At the beginning Morse walks past a large billboard advertising "Grimsby Pilchards" and featuring Diana Day, the beauty-queen character from the previous episode,"Trove". This billboard will feature in future episodes, too.
In the closing credits, after the cast of characters is listed, there are red letters in some of the crew's names. These letters spell out "Matthiola longipetala," the Latin name of the flower known as (evening) stock, which is one of the clues in the coat of arms Weiss was working on before he was murdered. The clues in the heraldry lead to the name of the murderer.
In order to focus Bunty's mind, Morse asks her "Beware the Jabberwock. What comes next?" The answer, from Lewis Carroll's poem Jabberwocky, is "Beware the Jabberwock, my son!/The jaws that bite, the claws that catch!/Beware the Jubjub bird, and shun/The frumious Bandersnatch!"
There are several literary references: the signpost reading Midwich, as in "The Midwich Cuckoos" by John Wyndham (and the plot, it is eventually revealed, involves "a cuckoo in the nest"), Karswell is the name of a character in M. R. James' story "Casting the Runes" (filmed as "Night of the Demon") and, at the College of Arms, Sir Hilary is said to be on holiday. This is Sir Hilary Bray who was abroad visiting Ernst Stavro Blofeld as told in Ian Fleming's "On Her Majesty's Secret Service".
When ghost hunter Stephen Fitzowen is giving Morse and Thursday background on the Blaise-Hamilton murders of 1866, he names Superintendent Cuff as one of the investigators into the mass murder. Cuff was the lead detective in the disappearance of The Moonstone in Wilkie Collins' Victorian novel of the same name.
Colin Dexter: Man leaning on and looking rather intently into display case during opening scene in museum.